Alcohol in your system is detrimental to any kind of fitness activity (except maybe on the dance floor). Here's how booze wreaks havoc on your regimen.

1. Slower Recovery
Hard workouts drain the glycogen stores (carbs stored in the liver and muscles) and leave your muscle tissue in need of repair. "Pouring alcohol into your system as soon as you finish stalls the recovery process," says Tavis Piattoly, R.D. High levels of alcohol displace the carbs, leaving your stores still 50 percent lower than normal even eight hours later, according to one study. Sip or snack on a combo of muscle-repairing protein and carbs (think low-fat chocolate milk or peanut butter on whole-wheat crackers) before tipping back.

2. Packed-On Fat
When booze is on board, your body, besides having to deal with the surplus of calories, prioritizes metabolizing the alcohol over burning fat and carbs. Alcohol also breaks down amino acids and stores them as fat. "For some reason this process is most pronounced in the thighs and glutes," says Piattoly. "Excessive alcohol consumption really chews up muscle in those areas." It also increases levels of cortisol (a stress hormone), which further encourages fat storage, particularly in your midsection.

3. Disrupted Sleep
Boozing also blows your muscle recovery and performance by sapping your sleep. In a study of 93 men and women, researchers found that alcohol decreased sleep duration and increased wakefulness (particularly in the second half of the night), especially in women, whose sleep time was decreased by more than 30 minutes over the night. "Disrupting the sleep cycle can reduce your human growth hormone output—which builds muscle—by as much as 70 percent," says Piattoly.

4. Depleted Water and Nutrients
Alcohol irritates the stomach lining, which can reduce your capacity to absorb nutrients (the reason you have an upset stomach after a few too many), says Brian R. Christie, Ph.D.—not to mention that alcohol makes you pee. For every gram of ethanol you suck down, you pump out 10 milliliters of urine (that's about 9.5 ounces for two beers). As little as 2 percent dehydration hurts endurance performance. And by the way, you can't rehydrate with a dehydrating drink (e.g., beer).